Previously, SemVer pre-release version was used to differentiate kind
of "branches", not allowing to cross pre-release on update.
According to SemVer spec, this should rather rely on the metadata.
New "keel.sh/matchPreRelease" annotation (or label) is added to disable
current keel behavior and consider the pre-release as a standard
SemVer versioning token.
For backward compatibility purpose, its value defaults to "true" if not
specified.
Closes#252
Also reviewed WatchRepositoryTagsJob tags handling.
Fixes#490
Joe Bowbeer
Neil Toledo
Aurélien Grenotton
Karolis Rusenas